Wait, if you did all the legal work, would it actually be legal to put one in a standard AR-15 to make it full auto?

If it is legal in your state to own full auto weapons,and the sear is one that was registered by the 1986 weapons deadline,then yes,you could put it in your lower.But I believe you would have to have the lower marked (Selective Fire)for it to be legal.
I have a friend that has 2 full auto converted AR's,he bought the auto sear kits back in the 1980's,and did all of the legal stuff thru a local Class 3 dealer.